1. Field of Invention
The invention relates to methods and systems for producing light from lower and higher energy activation sources. The invention also relates to systems and methods for broad band up conversion from the microwave and RF regime to electromagnetic radiation of higher photonic energy in the UV, VIS, and IR regime.
2. Discussion of the Background
Presently, light (i.e., electromagnetic radiation from the radio frequency through the visible to the X-ray wavelength range) is used in a number of industrial, communication, electronic, and pharmaceutical processes. Light in the infrared and visible range is typically generated from an electrical energy source which for example either heats a material to extremely high temperatures where black body emission occurs (as in an incandescent lamp). Light in the visible and ultraviolet range is typically generated by heating a gas to an electrical discharge where transitions from one electronic state of the gas atom or molecule occur with the emission of light. There are also semiconductor based light sources (as in light emitting diodes and semiconducting lasers) where electrons/holes in a material recombine to produce light emission.
Visible light is defined as the electromagnetic radiation with wavelengths between 380 nm and 750 nm. In general, electromagnetic radiation including light is generated by the acceleration and deceleration or changes in movement (vibration) of electrically charged particles, such as parts of molecules (or adjacent atoms) with high thermal energy, or electrons in atoms (or molecules). Both processes play a role in the glowing filament of incandescent lamps, whereas the latter process (electrons within atoms) occurs in fluorescent lamps.
The duality nature of light (or more generally electromagnetic radiation) is such that light is both a wave (characterized by a wavelength and amplitude) and a discrete parcel of energy or photon (characterized by its frequency times the Planck constant (denoted h). The higher the frequency the higher the quantized energy carried by the radiation. All energy above the visible is considered in many circumstances to be ionizing radiation as its photons carry sufficient energy to ionize matter.
For reference purposes, infra-red (IR) radiation just beyond the red end of the visible region; and, ultra-violet (UV) radiation has a shorter wavelength than violet light. The UV portion of the spectrum is divided into three regions: UVA (315-400 nm), UVB (280-315 nm) and UVC (100-280 nm).
Industrial lamps used in lighting applications cover the visible range of wavelengths for proper white perception. Thermal sources like heated filaments can be made of different type conductors, including W-filaments, halogen-protected W-filaments, and electrically induced high temperature plasmas (arc lamps).
The power (energy emitted per second) of a radiant source is frequently expressed in watts (W), but light can also be expressed in lumens (lm) to account for the varying sensitivity of the eye to different wavelengths of light. The derived relevant units are the radiance (luminance) of a source in W/m2 (lm/m2) in a certain direction per steradian (unit of solid angle) and the irradiance (illuminance) of a surface in W/m2 (lm/m2 or lux).
With the development of ultraviolet sources, ultraviolet radiation is being increasingly utilized for industrial, chemical, and pharmaceutical purposes. For example, UV light is known to sterilize media and is known to drive a number of photo-activated chemical processes such as the cross-linking of polymers in adhesives or coatings. Typically, ultraviolet sources use gas discharge lamps to generate emitted light in the ultraviolet range. The emitted light is then optically filtered to remove many of not all of the non-ultraviolet frequencies. Ultraviolet light can also be produced in semiconductor phosphors from the excitation of these phosphors from high energy sources such as, for example, X-ray irradiation.
With the development of infrared radiation sources, infrared radiation is being increasingly utilized for communications and signaling purposes. Typically, infrared sources use broad spectrum light sources referred to as glowbars to generate a broad spectrum of light centered in the infrared range or use lasers to emit very specific infrared wavelengths. For the broad band sources, the emitted light is optically filtered to remove many, if not all, of the non-infrared frequencies.
It is generally desirable to have devices, materials, and capabilities to convert light from one frequency range to another. Down conversion has been one way to convert higher energy light to lower energy, as used in the phosphors noted above. Up conversion has also been shown where lower energy light is converted to higher energy light. Typically, this process is a multi-photon absorption process where two or more photons are used to promote an excited electronic state in a host medium which in turn radiates at a wavelength of light that has a higher energy than the energy of the incident light which promoted the multi-photon absorption process. Both down conversion and up conversion have been studied and documented in the past.
Indeed, workers have studied the phenomenon of photoluminescence and fluorescence, which is the ability of certain solids to emit light when driven or charged by an external energy source. Many well-known phosphors and fluorescers are triggered by high-energy electrons or photons and emit photons of lower energy. There is a type of phosphor which can store energy for long periods of time in certain energy states. Relaxation from these energy states at a later time can be stimulated by less energetic photons. Relaxation from these energy states results in photon emission. The effect of this phenomenon is that energy is stored in the form of trapped electron-hole pairs for later use. Materials which exhibit this phenomenon will be referred to as electron trapping, or electron trapping phosphors, and materials in which emission of light is activated by infrared lumination are called infrared phosphors.
It has been recognized recently that certain infrared phosphors can actually operate at high speeds and are capable of converting pulsed infrared light to the visible range (violet through red). This “upconversion” occurs at the expense of the original charging illuminating light and can actually exhibit optical gain. It has been observed that phosphorescence can continue for as long as several days before a new short recharge are required.
Up conversion and down conversion of electromagnetic radiations are very relevant to various industrials fields. Photo-activated chemical reactions find broad use in the industry from catalyzing reactions to Bio-modulation of therapeutic agents. However, UV radiation suffers from a lack of depth of penetration in matter especially biological media, polymers and most solids). For this reason, UV based photo-initiation is limited by direct line of site which prevents volumetric applications.
UV has been limited to reactions taking place on the outer surfaces of materials may they be solids or liquids; organic or inorganic; biological organs, living tissues and composites thereof, structural composites, materials residing inside chemical tanks/reactors for food processing or hydrocarbon chains fractionation (to name a few examples).